Lombardi BPD not starting task

I had a Simulate User Task activity which I tried to assign to a List of Users - i.e. under Routing, the drop down was set to List of Users which contained 2 LDAP users and 1 user from the local repository (in this case tw_admin). When I ran this BPD, the user task simply didn't run. No errors in the SystemOut or SystemErr logs, nothing.  However, if I took the local repository user our of the List of Users it all worked fine. The key difference between the two users was their OU. Needs further investigation